Abstract : A backfire bifilar helix antenna is disclosed having two helically wound conductors made of coaxial cable wherein the two conductors comprise the shield portion of the cable. The coaxial cable of one of the conductors serves as a transmission line for supplying signals to the feed end of the antenna and has its center conductor connected to the shield of the other conductor cable. The nominal input impedance of the antenna may be adjusted to a desired value by conductive surface layers attached to the shield portion of the conductors at the feed end.